DEVELOPMENT OF A LANDSLIDE AND DEBRIS FLOW HAZARD MAP FOR THE OLD AND GRAND PRIX FIRES: SAN BERNARDINO NATIONAL FOREST

Approximately 140,000 acres were burned in San Bernardino National Forest during the October 2003 Old and Grand Prix fires. The fires were followed by intense storms in December of 2003, and near-record rainfall during the winter of 2004-2005. These storms resulted in widespread debris flows, landslides, and flooding, causing at least 18 deaths, and significant property damage. There is a need to systematically identify hazardous areas within these fires.

The Forest Service (in consultation with other agencies) is developing a landslide and debris flow hazard map for the Old & Grand Prix Fire areas. The method used in developing this map is intended as a prototype for application to Southern California Forests following future wildfires. Work is being coordinated with a study by USGS that expands on a debris flow hazard assessment conducted immediately after the fires (Cannon, S.H. Gartner, J.E., Rupert, M.G. and Michael, J.A. 2004). Post-2003 landslides and debris flows have been mapped on 2004 and 2005 air photos with limited field verification. The same features are also being mapped on 1972 air photos in order to evaluate the effects of the landslide-causing storms of 1968-1969. The aerial photo mapping will be compiled into a GIS coverage with associated database, which will be used in conjunction with other coverages (bedrock, geomorphic features, high resolution DEM, 3D imaging) to delineate hazard classes. Debris flow, landslide and rock fall hazards will be addressed. In addition, geomorphic terranes will be developed to predict sediment loads, which might be expected in similar adjacent watersheds in response to future fires.